This laboratory will continue to investigate the reaction sequence of the sodium and potassium ion pump through the kinetics of phosphorylation and dephosphorylation of the Na ion, K ion-ATPase in membranes derived from homogenates of guinea pig kidney. Phosphorylation will be studied both from (P32)ATP and radioactive inorganic phosphate. Dephosphorylation will be observed by chasing the phosphorylated enzyme with unlabeled substrate or by an excess of a chelator of Mg2 ion, which is required for phosphorylation. The following problems will be investigated. 1) Competition between Na ion and K ion for control of the active center for phosphorylation. 2) The sidedness of action of Na ion, K ion, and the inhibitor, ouabain in a preparation of tight vesicles. 3) The control of sidedness by phosphorylation. 4) The rapid kinetics of these reactions will be studied with a rapid mixing apparatus.